All human and simian immunodeficiency viruses contain a related central region gene termed vif, for viral infectivity factor, whose biological function is unknown. Although not absolutely required for viral replication in vitro, this gene is highly conserved within and between lentiviral species and has a demonstrated but obscure role in cell-free viral infectivity. This argues strongly for an important vif function in vivo. Little specific information about vif function is available, however, and it is not known whether vif acts at steps associated with virus attachment, at an early post-binding event, or in late (post- integration) processes. As a continuation of our studies on viral processes involved in entry and replication, the overall objective of this competitive renewal application is to define in molecular detail the biologic function of vif and to determine its role in viral pathogenesis in vivo. Specific aims are: (i) to define using biologically relevant HIV-1 and HIV-2 vif mutants, single- and multiple-round replication kinetics, natural target cells, and highly sensitive assays of viral nucleic acid and protein products, the precise stage in viral replication wherein vif exerts its biologic activity, and to determine what that action is; (ii) to evaluate directly the effect of the vif protein on HIV envelope maturation, oligomerization, function, and final virus assembly; (iii) to examine vif interactions with other viral accessory gene products (vpr, vpx) and the effect of other accessory gene defects in the context of vif deletion on viral replication; and (iv) to characterize the replication patterns and pathogenicity of wild-type versus vif- defective virus in the SIV SM/pig-tailed macaque (Macaca nemestrina) animal model system in order to define the role of vif in viral natural history and pathogenesis. The experimental approach to the analysis of vif function that we are proposing, including the use of viral clones derived from uncultured or single-passaged cells, highly sensitive assays for viral products at each stage of replication, and a specific focus on vif effects on native envelope conformation, is novel and reflects the particular interests and expertise of the participating investigators. The results of these studies are expected to provide new insights into virus entry and replication, and in addition, information that is relevant to the identification of additional targets for antiviral therapy and alternative strategies for vaccine development.